Semiconductor lasers play an important role in optical communication systems, particularly in optical communication systems that employ high digital data rates. It is desirable that these semiconductor lasers not only have a small size but also offer various features that make them attractive for use in a number of different applications. One such feature that is desirable in a semiconductor laser is wavelength tunability. Wavelength tunability, which is available in some types of traditional semiconductor lasers, allows a laser to be tuned to a first wavelength for use in a first application and tuned to a different wavelength where needed for use in a different application.
Traditional tunable wavelength lasers can be classified under two broad categories. A laser belonging to the first category is typically referred to in the art as a Distributed Bragg Reflector (DBR) tunable laser and a laser belonging to the second category is typically referred to as an external-cavity tunable laser. Each of these two types of tunable lasers has its strengths and weaknesses but neither one can be used in a universal manner for all applications.
DBR based tunable lasers generally are compact in size and are often used in various direct-detection based communications systems. However, these lasers have two significant shortcomings: 1) fabrication issues, and 2) an undesirable broad spectrum emission characteristic. While fabrication issues can be mitigated to some extent by improvements in fabrication techniques, the broad spectrum emission characteristics of DBR based lasers often prevents their use in coherent communication systems requiring narrow emission linewidths for proper operation.
External-cavity tunable lasers have inherently narrow emission linewidths making them suitable for coherent communication systems as well as some other systems such as direct detection systems. However, these lasers can be quite complex to fabricate as well as to use. For example, it is difficult to maintain long term stability in an external-cavity tunable laser in comparison to a DBR based tunable laser. Furthermore, both DBR tunable lasers and external-cavity tunable lasers suffer from cost structure challenges that limit their broad use in optical communication systems.
It is therefore desirable that such issues in traditional tunable wavelength lasers be addressed.